The public and decision makers at the corporate, utility and
governmental levels need to be informed about the current status of
solar technology and the dramatic technical and cost improvements it has
undergone during the past few years. This can best be accomplished by
demonstrating the effectiveness of photovoltaics in some exciting new,
eye catching ways. I think that PV central power station, base load
generation of electricity lies far into the future. One might even argue
that intermittent, low voltage DC electricity supplied by photovoltaics
is not well matched to the present central power grid. Power
conditioning to convert to higher voltage AC accounts for typically half
the cost of a complete PV power station, and storage adds substantially
more to the cost and operating complexity. On the other hand, the
present electrical distribution system is attempting to meet needs that
might be much better served by photovoltaics. Let me suggest four areas
in which I believe that the use of PVs could become cost effective and
begin to displace conventionally supplied electricity during the 1990s:
air conditioning, water pumping, chemical hydrogen production and
transportation.

Air Conditioning

The most rapidly growing market for electricity in the US and many
tropical countries today is for space cooling. Meeting this demand with
centrally generated power is very expensive because of the high capital
cost and operating expenses needed to meet short term peak demands. If
ever there were a match between the availability of solar isolation and
the local demand for solar electricity, this is it. By avoiding the need
for expensive storage and by competing in the most expensive, peak
demand conventional market, central solar thermal electric stations,
supplemented by small amounts of natural gas are able to provide
competitively priced power in Southern California and other tropical
countries.

Alternatively, air conditioning demand might be met on-site by PVs
powering a high efficiency (either brushless or reactance) DC motor
driving a conventional refrigerator compressor. This strategy does not
need expensive energy storage, avoids losses during transmission and DC
to AC conversion and competes in the most expensive market. By avoiding
about one quarter of the losses, one can have a proportional decrease in
the size and cost of the PV array, and by eliminating expensive power
conditioning equipment, lower significantly the capital cost of solar
air conditioning. If one wished to extend the cooling time of the system
into the evening hours, it would be less expensive to add either a
backup AC motor or an AC to DC converter that operated from the
conventional grid than to build a complex electrical storage system. An
even simpler and cheaper cooling strategy involves PV powered DC attic
ventilation fans that could greatly reduce building air conditioning
demand. Clearly, the most cost effective PV powered air conditioning
system would combine the cooling fan and a relatively smaller heat pump
with the most efficient motor available and a modest PV array. Such a
total solar space cooling system would be ideally suited to large flat
roofed commercial and manufacturing buildings whose owners are more
likely than householders to invest in a new technology.

Similarly, PVs are ideally suited for powering the small motors
used to pump heat exchange fluids in solar hot water and pool heaters.
In order for any of these uses to penetrate the market,it will be
necessary to develop complete PV powered that are easily installed by
the home owner or a contractor. After all no one, would buy a solar
powered calculator if it were necessary to buy the components separately
and wire them together before balancing ones check book.

Water Pumping

Let's examine a second familiar application from a different
perspective. Solar powered water pumps have long seemed attractive
applications for PV electricity because the demand for delivered water
is less constrained by the intermittent nature of solar power, and water
pumping is often needed far from existing power lines. We tend to think
of solar water pumping as small scale, of the order of a kilowatt or so,
but what if we used PV powered pumps to move irrigation water for some
major projects and other arid regions? One could envision distributed
solar arrays built on top of the aqueducts, many of which should be
covered to reduce evaporation losses anyway, and along the adjacent
right of way. This arrangement if carefully engineered could
significantly reduce the costs of structural supports and land
acquisition of the PV system. Once again the close proximity of the
electricity source to the end use reduces line losses, and the use of
efficient DC powered pumps eliminates conversion losses and equipment
costs relative to central station alternatives. A plan to transport and
desalinize ground water in Australia by long distance pipelines using PV
powered DC driven pumps has been proposed by Lonrigg[1]. He has also
carried out a general analysis of this approach as well[2]. California
will be negotiating new electricity contracts for pumping water in the
1990s and is expecting to pay much higher prices. Such a strategy might
significantly reduce the pressure to construct additional fossil fuel
power plants.

Chemical Hydrogen

Production

One area for which the low voltage, DC output of photovoltaics is
ideally suited is in the electrolysis of water to produce hydrogen and
oxygen. As in plant photosynthesis, the desired product becomes the
storage medium so that solar intensity variability is not a serious
concern. Hydrogen is an important feedstock for the chemical industry
particularly for the manufacture of energy intensive chemicals such as
ammonia, NH3. It is also produced in major quantities for the petroleum
refining industry, and large quantities are utilized in the food
processing industry to hydrogenate edible oils. The potential market is
surprisingly large as the energy content of US produced hydrogen is
nearly 2.5 per cent of total energy use[3] and since it is typically
stripped from methane, CH4, significant amounts of CO2 are released in
the process. According to an analysis by Joan Ogden and Robert Williams,
high purity, merchant hydrogen, generated by photovoltaic driven
electrolysis could begin to compete economically with current sources
for this market by the middle of the 1990s[3]. The economics are
enhanced by the co production of another important industrial gas,
oxygen, and by the inherent clean air benefits of such a solar based
industry. What is needed is for a manufacturer to assemble and market an
integrated PV hydrogen production system consisting of a solar array, an
electolyzer and gas storage system.

Transportation

The transportation sector is a significant contributor of
greenhouse gases and to regional and local air pollution. The
world's 500 million motor vehicles each year add more than 800
million tons of carbon as carbon dioxide or 15 per cent of the fossil of
fuel contribution[4]. Furthermore, this amount has been increasing both
in absolute terms and as a percentage of the global total. As the major
source of carbon monoxide which increases the atmospheric lifetime of
the methane, and as a significant producer of tropospheric ozone and
hydrocarbons, vehicles contribute additional gases to the greenhouse
effect.

The principal technologies proposed to replace our carbon fueled
transportation system are hydrogen powered internal combustion engines
or fuel cells and electric motors. To address the greenhouse problem,
both electricity and hydrogen would need to be produced by non-fossil
fuel powered systems. The available technologies include photovoltaics,
solar thermal, hydropower, wind, biomass and nuclear power. Because of
their modularity, simplicity and ability to be located on site,
photovoltaics are especially attractive for both options.

Electric vehicles are an especially attractive option if the
problem of onboard storage of energy can be solved. The low voltage DC
output of solar cells is perfectly matched to any imaginable battery
system without the need for any power conditioning equipment. One could
readily envision a fleet of small commuter vehicles that could be
recharged during the day by PVs mounted on the roof of covered parking
lots and commercial building. Additional PVs on the roof of homes could
charge spare storage batteries that could be readily substituted when
needed. While battery technology has been slow to improve, major
advances have been recently made in the design and performance of the
vehicles themselves as was recently illustrated by GMs Impact. Also the
recent announcement by Isuzu that it had developed a new, low cost
electricity storage system based upon a super high energy density
capacitor may make electric vehicles practical in the very near term.

Ogden and Williams have proposed that PV produced hydrogen become a
transportation fuel in the sunny Southwestern United States. The
advantages for local air quality, acid rain and the greenhouse effect
are obvious since the principle product of hydrogen combustion is water
and a few nitrogen oxides. These latter emissions should be readily
controlled by a one way catalyst since there are no hydrocarbons or
carbon monoxide to complicate pollution control. Ultimately, they might
be eliminated entirely by using hydrogen to power a fuel cell.
Currently, BMW, Siemens and the Bavarian government are building a 600
KW PV-hydrogen production system in Germany to fuel test vehicles. Both
BMW and Daimler Benz have prototype, hydrogen fueled internal combustion
cars, and Daimler will begin production of a hydrogen powered urban bus
in the near future. Additional work needs to be done on hydrogen storage
systems, but the basics of the technology are well understood. Once
again, end use efficiency plays a synergistic role with a solar
technology since greater vehicles fuel economy reduces the hydrogen
storage problem and the size and cost of the PV array per vehicle.

Ogden and Williams estimate that if photovoltaics get into the
price range of $ 0.40/peak watt, that hydrogen as a fuel would be
competitive with gasoline at $ 1.50 - 2.00/gallon around the turn of the
century. It could begin to make inroads by being utilized in fleet
vehicles and buses. Unlike other synthetic fuels programmes, because of
the modular nature of the production facility, it is possible to get
started in producing PV hydrogen in a 5 to 10 MW facility for as little
as $4 to 12 million[4].

The displacement of fossil fuels by hydrogen represents one of the
few strategies that could simultaneously address energy security
concerns, the severe photochemical smog problem of sunny regions, acid
deposition and the green house effect.